A Gaussian surface is a hypothetical imaginary surface which can be used, along with Gauss’ Law, to calculate the electric field of a Continuous Charge Distribution.
Definition
A gaussian surface must be a closed, 3-dimensional surface. While it can be theoretically any such shape, a Gaussian surface is most useful if it matches the field symmetry. Why?
The electric flux on on complex surfaces can be simplified if have the surface either being perpendicular or parallel to the electric field, completely. That is precisely what a gaussian surface intends to do.
Most commonly, cylindrical and spherical gaussian surfaces are used.